Throughout most of North America as well as in other parts of the world, the three digits "911" have been selected as the Emergency Services Notification telephone number. Other countries use different sequences of numbers for the same purpose. For example, "111" is used in the United Kingdom and "999" in other parts of Europe. In areas where emergency services notification has been implemented, telephone users can call for any type of emergency assistance by dialing the ESN telephone number. In order for this service to be available, the Public Switched Telephone Network (PSTN) supplier in the area must route all such calls to an appropriate Call Answering Center. Many jurisdictions have installed enhanced systems that include technologies for reporting the calling party's Directory Number (DN) directly to the Call Answering Center. This information is then cross referenced to a location database which identifies the geographical location of the calling party.
The geographical information is used to dispatch the emergency services crew to the caller's location. In most cases the geographical database depends on a consistent and rigorously applied Civic Addressing System (CAS) which supplies a unique address for each possible originating telephone number. In practice, the CAS scheme is rarely implemented with 100% accuracy and often the CAS address label or sign is either misplaced or obstructed from view. In addition, mid to large sized commercial, industrial, educational and governmental institutions typically cannot participate in the ESN program as effectively since their internal PABX telephone systems have no practical way to report location information to the PSTN. An industrial site, for instance, may have hundreds of telephone instruments located in various buildings over several hectares yet a "9-1-1" call from any of them may identify only one location.
As a result of not quickly recognizing where the emergency site is located, either because the CAS addressing is incorrect or the address marker is not visible from the roadway or the emergency is declared from within a campus of buildings, emergency response time, which is of the essence in such situations, is undesirably increased. There exists, therefore, a need to supplement the emergency response tools to more effectively direct emergency responders to the actual emergency site.